In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium, to form characters and image. The ink-jet printing methods have been rapidly spread because of their various advantages such as easiness of full coloration, low costs, capability of using a plain paper as the recording medium, non-contact with printed images and characters, etc.
As to the ink-jet printing methods, for example, W 01/096483 discloses an ink for ink-jet printing which contains a colorant formed by coating a pigment and/or a dye with a polymer, water, and at least one compound selected from the group consisting of an acetylene glycol-based surfactant, an acetylene alcohol-based surfactant, a glycol ether and a 1,2-alkylene glycol, exhibits excellent dispersion stability and ejection stability, and is capable of providing images which are excellent in color developability, etc.
JP 2003-096345A discloses a pigment-containing ink for ink-jet printing which is composed of a polymer emulsion containing polymer fine particles into which a water-insoluble or hardly-soluble colorant is included, and which also contains at least a wetting agent such as glycerol, 1,3-butanediol, 1,6-hexanediol and 1,5-pentanediol as well as a polyol or a glycol ether having 8 to 11 carbon atoms, an anionic or nonionic surfactant, a water-soluble organic solvent and water, wherein the ink has a viscosity of 5 mPa·s or more as measured at 25° C., is excellent in ejection stability and storage stability, and is capable of providing printed images having good quality, etc., even upon high-speed printing on plain papers.
JP 2004-91590A discloses a water-based pigment dispersion containing a pigment, a water-soluble organic solvent and a copolymer resin obtained from a styrene monomer and an acid group-containing monomer wherein a content of the styrene monomer component in the copolymer resin is 50 to 90% by weight, and the dispersion further contains inorganic oxide fine particles in an amount of 0.01 to 10% by weight on the basis of the weight of the pigment.
However, the above conventional water-based inks are still unsatisfactory in optical density when conducting one-pass ink-jet printing on plain papers using the inks.